Parts made of C--C composite material are manufactured by making a fiber preform of a shape that is close to that of the part to be manufactured and by densifying the preform by inserting a matrix into the pores of the preform.
The preform is made from a base fiber fabric, for example by winding a strip of base fabric in superposed layers or by stacking or laying plies of base fabric. By way of example, the base fiber fabric may be a felt, a woven cloth, a knit, a braid, a unidirectional sheet of yarns, tows, or strands, or indeed a multilayer fabric made up of a plurality of superposed unidirectional sheets having different directions and bonded to one another by light needling.
Document GB-A-1 447 029 describes making such a multilayer base fiber fabric. The various needled-together layers can be impregnated with a thermoplastic resin and compressed in order to obtain a consolidated fiber fabric suitable subsequently for making composite materials.
The preform can be densified with a carbon matrix by chemical vapor infiltration and/or by a liquid technique. Chemical vapor infiltration consists in placing the preform in an enclosure into which there is inserted a gas containing one or more gaseous precursors of carbon. These are typically selected from alkanes, alkyls, and alkenes, with commonly used precursors being methane and/or propane. The conditions of temperature and pressure inside the enclosure are determined so as to enable the gas to diffuse into the pores of the preform and to form a deposit of pyrolytic carbon on the fibers by decomposition of the carbon precursor(s) it contains. The liquid technique consists in impregnating the preform with a composition containing a carbon precursor in the liquid state, e.g. pitch or a resin having a non-zero coke content. The precursor is transformed by heat treatment that gives rise to coke. Thus, for example, document WO-A-92/01648 describes a method of manufacturing C--C brake disks which consists in associating plies of fiber fabric with a polymerizable carbon precursor and in performing heat treatment, at least in an initial stage, under pressure so as to consolidate the preform and transform the precursor, thereby obtaining the desired C--C material.
Nevertheless, to ensure cohesion of the preform and the ability of the manufactured part to withstand delamination, it is desirable to bond together the layers or plies constituting it. Such bonding is advantageously performed by needling, as described for example in documents U.S. Pat. No. 4,790,052, FR-A-2 626 294, and FR-A-2 726 013.
Another method, described in document WO-A-91/01397consists in forming a fiber structure, e.g. by stacking plies, in compressing the fiber structure so as to obtain a preform of a shape that is close to the shape of a part to be manufactured, and in maintaining the preform in the compressed state by a series of needling operations performed from both sides of the preform over its entire thickness, or by stitching using a thread passing through the preform.
Yet another method, as described in document FR-A-2 619 104, consists in performing three-dimensional impregnation by superposing and needling together plies of fiber fabric that is impregnated with a composition comprising a carbon-precursor resin. The pre-impregnated piece is then shaped under pressure and subjected to heat treatment to carbonize the precursor and obtain a carbon matrix composite.
To manufacture disk brakes out of C--C composite material, annular fiber preforms are generally made by needling plies of a base fiber fabric comprising a woven cloth or a laminate comprising a plurality of unidirectional sheets of yarns, tows, or strands, optionally associated with a fiber web or a felt. The plies are needled one at a time so as to obtain needling of a predetermined density in the thickness of the preform. The preforms are usually densified by chemical vapor infiltration, with the needled preforms having sufficient mechanical strength to avoid the need to use support tooling.
Brake disks made in this way are used for aircraft disk brakes or for the brakes of F1 racing cars where they give entire satisfaction.
In other applications such as braking rail vehicles, industrial vehicles, or private cars, where conditions of use are generally much less severe, tests performed by the Applicant using the same disks have not been so satisfactory. In particular, there have been observed the appearance of undesirable vibration, of irregularities in braking torque, and sometimes of wear greater than that which could have been hoped for. In addition, manufacturing costs are high and difficult to make compatible with generalized use on rail vehicles or on industrial vehicles or on mass-produced cars.
An object of the present invention is to remedy those drawbacks and, in most general terms, it proposes a method that is suitable particularly, but not exclusively, for manufacturing brake disks for various applications.
In particular, an object of the invention is to provide a method enabling brake disks made of C--C composite material to be obtained that present good mechanical and tribological properties and that are suitable for being used under various conditions without generating undesirable vibration and without presenting unacceptable wear.